Multiple drivers of large‐scale lichen decline in boreal forest canopies

Thin, hair-like lichens (Alectoria, Bryoria, Usnea) form conspicuous
epiphyte communities across the boreal biome. These poikilohydric
organisms provide important ecosystem functions and are useful indicators
of global change. We analyse how environmental drivers influence changes
in occurrence and length of these lichens on Norway spruce (Picea abies)
over 10 years in managed forests in Sweden using data from >6000
trees. Alectoria and Usnea showed strong declines in southern-central
regions, whereas Bryoria declined in northern regions. Overall, relative
loss rates across the country ranged from 1.7% per year in Alectoria to
0.5% in Bryoria. These losses contrasted with increased length of Bryoria
and Usnea in some regions. Occurrence trajectories (extinction,
colonization, presence, absence) on remeasured trees correlated best with
temperature, rain, nitrogen deposition, and stand age in multinomial
logistic regression models. Our analysis strongly suggests that industrial
forestry, in combination with nitrogen, is the main driver of lichen
declines. Logging of forests with long continuity of tree cover, short
rotation cycles, substrate limitation and low light in dense forests are
harmful for lichens. Nitrogen deposition has decreased but is apparently
still sufficiently high to prevent recovery. Warming correlated with
occurrence trajectories of Alectoria and Bryoria, likely by altering
hydration regimes and increasing respiration during autumn/winter. The
large-scale lichen decline on an important host has cascading effects on
biodiversity and function of boreal forest canopies. Forest management
must apply a broad spectrum of methods, including uneven-aged continuous
cover forestry and retention of large patches, to secure the ecosystem
functions of these important canopy components under future climates. Our
findings highlight interactions among drivers of lichen decline (forestry,
nitrogen, climate), functional traits (dispersal, lichen colour,
sensitivity to nitrogen, water storage), and population processes
(extinction/colonization).